Ultrasonic camera tube

ABSTRACT

An ultrasonic camera tube for converting a two-dimensional pattern of sound pressure into an electrical signal which may be displayed by a television picture tube as an equivalent light pattern. The camera tube includes a piezoelectric transducer for converting the sound pressure pattern into an electrical charge pattern. A photocathode mosaic is deposited on the inner face of the transducer. The photocathode mosaic is flooded with light to achieve photoemission of electrons. The charged mosaic elements discharge in vacuum through an adjacent grid and proceed onward to charge the capacitive storage elements on a mosaic screen adjacent to the grid. An electron beam then scans the elements of both cathode and screen mosaics to return them to their original states and also provides the signal readout. Readout is obtained by sensing the current taken from the elements of the mosaic screen during scanning wherein the sensed current is used to reproduce the sound image on a television tube.

United States Patent [1 1 Sackman ULTRASONIC CAMERA TUBE [75] Inventor: George L. Sackman, Carmel Valley, Calif.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

[22] Filed: Apr. 2, 1971 [21] Appl. No.2 130,697

OTHER PUBLICATIONS A Study of Acoustic Image Converters, Soviet Physics-Acoustics, Vol. 4, No. 1, pp. 72-83, 1958.

The Ultra-Sound Image Camera, IEEE Proceedings, Vo1.l10, No. 1, pp.16-28, 1963. The Ultrasound Camera-Recent Considerations, Ultrasonics, pp. 15-20, Jan. 1966.

An Ultrasonic to Electronic Image Converter Tube for 1 June 26, 1973 Operations at 1.20 Mc/S, The Radio & Elec. Eng., V01. 31, No.3, pp. 161-180, March 1960.

An Electron Beam Acoustic Image Converter, Operating in a Pulsed Transceiving Mode, IEEE Trans. on Sonics & Ultrasonics, Vol. SU-16, No. 3, July 1969, pp. 94-97, 99, 101 & 102.

Primary ExaminerCarl D. Quarforth Assistant Examiner-P. A. Nelson Attorney-R. S. Sciascia and Charles D. B. Curry [5 7] ABSTRACT .An ultrasonic camera tube for converting a twodimensional pattern of sound pressure into an electrical signal which may be displayed by a television picture tube as an equivalent light pattern. The camera tube includes a piezoelectric transducer for converting the sound pressure pattern into an electrical charge pattern. A photocathode mosaic is deposited on the inner face of the transducer. The photocathode mosaic is flooded ,with light to achieve photoemission of electrons. The charged mosaic elements discharge in vacuum through an adjacent grid and proceed onward to charge the capacitive storage elements on a mosaic screen adjacent to the grid. An electron beam then scans the elements of both cathode and screen mosaics to return them to their original states and also provides the signal readout. Readout is obtained by sensing the current taken from the elements of the mosaic screen during scanning wherein the sensed current is used to reproduce the sound image on a television tube.

7 Claims, 10 Drawing Figures PATEN IEU JUN 2 6 I973 SHEEI' 1 052 INVENTOR. GEORGE L. SACKMAN AT ORNEY PATENTEDJUN 26 I975 ELECTRON EMISSION SHEET 2 BF 2 0 c VOLTAGE GEORGE L. SACKMAN A B 2 |.O n l l 1 INVENTOR have/fa ATTORNEY ULTRASONIC CAMERA TUBE The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalities thereon or therefor.

The present invention relates to an ultrasonic camera tube and more particularly to a technique for converting a two dimensional pattern of sound pressure into an electrical signal for displaying on a television picture tube as an equivalent light pattern.

One of the present techniques forv converting a sound pattern into an equivalent light pattern involves the use of a cathode ray tube with the face being a thin piezoelectric plate. With this technique the outer surface of the plate is in contact with the surrounding medium wherein the sound pressure on the face causes local strains, thereby generating electric potentials on the inner face. These electric potentials are with respect to the outer face having an electrode at ground potential. The electron beam in the tube is caused to scan the inner face. Charging currents to the outer electrode and/or a change in secondary emission at each point in the scan raster (caused by the various piezoelectric potentials) are detected to produce an electrical signal. This signal is then used to produce a television image of the pattern using normal television techniques.

One of the primary disadvantages of this present technique is that no storage of sonic .or electrical energy is provided by the piezoelectric plate except for a low Q mechanical resonance. The practical result is tion other than the point at which the scanning beam strikes is entirely lost. Since the scanning beam strikes only a small fraction of the face at any time, most of the impinging energy is not converted into electrical signals and the sensitivity to weak sound is very poor.

Another practical disadvantage is that due to lack of sound signal storage, continuous sound is required to produce a continuous converted image. This continuous sound gives rise to standing waves and backseattered energy between the sound source and the tube which further degrades the resolution and sensitivity of the system. Furthermore, synchronization between the sound transmitter frequency and the receiver scan is required in order to prevent moving "bars in the display. An additional disadvantage is that no amplification of the piezolectric potential is available before scanning so that the shot noise of the election beam and secondary emission current limits the sensitivity to weak sound;

The present invention overcomes the above mentioned disadvantages of existing techniques and also provides a tube having certain other advantageous features.

Accordingly, an object of present invention is to provide a technique for converting a sound energy pattern into an equivalent light pattern that is sensitive and has good resolution.-

Another object is to provide an ultrasonic cameratube that has a sound memory thereby eliminating the necessity of continuous sound transmission and providing a relatively noise-free system.

Briefly, the present invention comprises an ultrasonic camera tube for converting a two-dimensional pattern of sound pressure into an electrical signal which may be displayed in a television picture tube as an equivalent light pattern. The camera tube includes a piezoelectric transducer for converting the sound pattern into an electrical pattern. A photocathode mosaic is deposited on the inner face of the transducer. The photocathode mosaic is flooded with light to achieve photoemission of electrons. The charged mosaic elements discharge in vacuum through an adjacent grid and proceed onward to charge the capacitive storage elements on a mosaic screen adjacent to the grid. An electron beam then scans the elements of both cathode and screen mosaics to return them to their original states and also provides the signal readout. Readout is obtained by sensing the current taken from the elements of the mosaic screen during scanning, wherein the sensed current is used to reproduce the sound image on a television tube.

The light used to flood the photocathode mosaic is preferably gated to be on only when the sound echo signal is to be received by the tube. By gating it in this manner, sound received from the foreground and background is virtually eliminated thereby providing improved resolution and sensitivity.

The electron beam that is used to return the capacitors formed bythe photocathode mosaic and the mosaic screen to their original state is adjusted to an anode voltage at the electron gun above the critical potential for secondary emission from the screen material. Part of the electrons strike the mosaic screen and return those elements to their original positively charged state that sound energy impinging upon the plate at any locaby virtue of a secondary emission ratio that is greater than unity. The remaining part of the electron beam is slowed down by the grid to below the critical potential, thereby depositing electrons on the photocathode mosaic and returning these elements to their original uncharged state.

Other objects, advantages, and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a sound to light conversion system with which the present invention is concerned;

FIG. 2 is a schematic diagram of the ultrasonic camera of the present invention that is used as part of the system components of FIG. 1;

FIG. 2A is an illustration of an alternative method for making the mosaic screen of FIG. 2;

FIG. 3 is an equivalent circuit of a single mosaic cell including the common electron gun of the camera tube of FIG. 2;

FIG. 4 is a curve illustrating the electron emission characteristic of the photocathode mosaic of FIGS. 2 and 3 in response to light impinging thereon;

FIG. 5 is an equivalent circuit of only the single mosaic cell of FIGS. 2 and 3;

FIG. 6 is a curve illustrating the voltage of the photocathode mosaic during the receipt of a sound echo signal;

FIG. 7 is a curve illustrating the charging and discharging of the mosaic screen capacitors during the receipt of a sound echo signal;

FIG. 8 is a curve illustrating the secondary emission ratios of the screen and cathode surfaces as a function of the anode voltage of the electron gun; and

FIG. 9 is a schematic diagram of an alternative embodiment of the present invention.

In FIG. 1 is illustrated a schematic diagram of a sound to light conversion system 11 with which the present invention is concerned. This system includes a sound transducer 13 that transmits sound as illustrated by transmitted sound waves 15. In accordance with the present invention the signal produced by the sound transducer has a frequency of about one megahertz Hz) and a time duration of about 0.1 millisecond (10 second) wherein each sound burst has a total of about 100 cycles of sound (10 X 10 100). These transmitted sound waves impinge upon an object 17 and are then reflected by the object as illustrated by reflected sound waves 19. These reflected sound waves are collected and then focused by sound lens 21. Sound lens 21 focuses the incoming reflected sound waves to form a sound pattern 23 on the exterior face of transducer element 25. Transducer element 25 is made of piezoelectric or electrostrictive material and therefore produces an electric charge that is proportional to the sound pressure at each point. Conversion device 27 then converts the electric charges on the back surface of transducer element 25 into amplified and useful electrical signals. Transducer element 25 and sound conversion device 27 together comprise the ultrasonic camera tube 29 of the present invention which is hereinafter described in detail. The electrical information from sound conversion device 27 is converted to light information by electrical conversion device 29. The light information from electrical conversion device 29 is projected onto a screen 31 to form light pattern 33 which is a replica of object 17. System of the type described in FIG. 1 may be used in virtually any medium, but it is well adapted for and is frequently used for under-water observation. In a system of this type the following relationships should hold to provide a light pattern 33 that is suitable replica of object 17. These relationships are:

R/)t l Where it is the wave length of the sound produced by transducer 13, D is the diameter of lens 21, L is the size of the object, and R is the distance between the object and the lens.

In FIG. 2 is illustrated a schematic diagram of the various elements of the ultrasonic camera tube 29 of the present invention. The outside surface of transducer element 35, which is equivalent to the exterior face of transducer element 25 of FIG. 1, is normally exposed to water and the focused sound information from lens 21 of FIG. 1 impinges upon the surface thereof to form a sound patern of the object 17.

All elements of ultrasonic camera tube 29 are preferably in a vacuum except for the outer surface of transducer element 35. The light source 36 is located either inside or outside of the vacuum in such a way as to illuminate photocathode mosaic 41 either periodically or continuously as hereinafter explained. Transducer element 35 of ultrasonic tub 29 consists of piezoelectric or electrostrictive plate 37, outer electrode 39 and photocathode mosaic 41. Outer electrode 39 is preferably a deposited coating that has both electrical and sound conducting qualities. The sound conducting quality applies the incoming sound to piezoelectric plate 37 and the electrical conducting quality provides a reference potential or ground. The piezoelectric plate 39 may be of ceramic or quartz material, for example, that produces an electric charge that is proportional to the deformation of the material that is in turn proportional to the sound pressure at discrete points on the surface thereof. The photocathode mosaic 41 comprises a material that has photoemission characteristics or is capable of giving off electrons when illuminated by light. This photocathode mosaic is deposited on the inside surface of plate 39. The mosaic comprises a plurality of islands or elements 43 that are insulated from each other. This may be achieved by depositing a photoemissive material through a mask pressed adjacent to the inside surface of plate 39 or by removal of deposited photoemissive material by etching or mechanical techniques to form elements 43 that are insulated from each other.

Preferably the light from source 36 is of sufficient in tensity to cause saturation or maximum electron photoemission of elements 43 of photocathode mosaic 41 as indicated at point A of FIG. 4. Light flooding of the elements results in photoemission and formation of a space charge or electron cloud between mosaic 41 and grid 45. However, the light flooding of elements 41 may be at less than saturation, such as at point B of FIG. 4. It is also to be understood that the intensity of the light may be modulated between points C and D, for example, to superimpose information upon the incoming sound information. This could provide a reference wave for recording holograms, for example. Normally, however, light from light source 36 will be kept above saturation or at a constant intensity when below saturation. With the flood light on at any given intensity above saturation the electron emission from each of the elements will be the same provided the piezoelectric potential of each element is the same. This condition will occur when the sound pressure at each point on electrode 39 is the same or if no sound is being received. However, when a sound pattern is formed on the surface of electrode 39, then the piezoelectric potential of the affected element 43 will correspond to the intensity of a point in the impinging sound pattern. The electron emission of each element that is due to light is arrested by the formation of the space charge between cathode and grid. However, superimposed upon this is the increased electron emission due to localized increases in electrical potential caused by the piezoelectric effect. Therefore, there will be no further electron emission from the elements not receiving sound information whereas there will be an increased electron emission from the elements that correspond to the pattern imposed on the outside surface of electrode 39.

The ultrasonic tube 29 of FIG. 2 also includes grid 45 comprising, for example, a conducting mesh or a mosaic screen 47. Mosaic screen 47 comprises a screen made of nonconducting filaments 44 wherein beads or elements 49 of conducting material are deposited on each intersection. Elements 49 of mosaic screen 47 correspond to elements 43 of photo cathode mosaic 41. The ground for elements 49 is ring 59 as hereinafter explained.

An alternative method for forming a mosaic screen 47 comprises the use of a plurality of rows of parallel and spaced apart capacitor arrays. A capacitor array 46 is illustrated in FIG. 2A and comprises a wire 48 covered by glass insulation 50 and spaced apart cylinders or elements 52 of electrical conducting material covering preselected regions of the glass insulation. A plurality of rows of these elements 52 correspond to elements 43 0f photocathode mosaic 41 and are preferably in longitudinal alignment therewith. The core wires are connected to ground and the wires and elements together constitute individual capacitors referred to as C, in FIGS. 3 and 5.

In FIG. 3 is illustrated the quivalent circuit of a single mosaic cell and common electron beam source. Voltage E, is the piezoelectric potential produced as a result of local sound pressure on an individual photocathode element 43 of transducer 35. Capacitance C, is the clamped capacitance of the transducer 35 associated with the single mosaic cell area. R, includes any electrical resistance between the individual photocathodes 43 and circuit ground. V, is the bias voltage at the grid 45. Capacitance C, is the capacitance between element 49 and ring 59. Resistance R, is an external resistor between the ring and ground wherein the voltage across this resistor during scanning readout is proportional to the charge on the element 49 in question. V, is the voltage applied to the anode of gun 51 and to collector ring Referring to FIGS. 3, 5, 6, and 7, photocathode 43 will emit electrons which will be collected by element 49 when photocathode 43 is negative with respect to grid 45 This is illustrated in FIG. 6, where V, is the grid potential and when E, (the potential on element 43) is below V,,. In FIG. 7 this is further illustrated wherein the reference numerals correspond with the voltage E, of FIG. 6. From this it can be seen that in operation the sonar pulse echo, which will typically be a sinusoidal signal (as shown in FIG. 6) having a finite time duration of about 100 cycles, will gradually charge up capacitor C, (by loss of electrons from the photocathode elements 43) and discharge capacitor C, (having a positive initial D. C. level and discharging toward ground by collecting the electrons emitted from the photocathode). This process will continue until the potential on C, equals the peak value (E,) on photocathode elements 43. These charges will remain on these capacitors C, and C, until returned to their original states as hereinafter described. These charged capacitors constitute the memory of the system.

After all of the equivalent capacitors C, and C, of the mosaics are charged by the echo signal, then all of the elements (equivalent capacitors C, and C,) are scanned by a destructive readout process. Destructive readout constitutes discharge of all the capacitors. Resistor R, is common to all capacitors C, and is included in external circuits to measure the current passing from capacitors C, during destructive readout. The timevarying voltage across resistor R, is the electrical signal representing the sound pattern and these electrical signals are used for operation of the video display device. It should be noted that the electron beam scan is synchronized with the TV display by usual techniques. After each destructive readout the system is then ready for the next sonar pulse echo.

Grid 45 is employed to achieve amplification and rectification. Ideally, grid 45 is at ground potential (Class B operation) so that electrons would pass therethrough during each negative 1% cycle at the cathode as previously explained with reference to FIGS. 3, 5, 6, and 7. However, some of the positive potential of mosaic screen 47 (plate) will be seen on the photocathode mosaic 41 (cathode) side of the grid. Therefore, to have it so that the cathode sees zero or ground potential, the grid must be biased somewhat negative. From this it can be seen that the cathode-to-grid portion of the tube provides a diode effect for plate rectification. Voltage amplification is achieved because grid 45 makes it possible to isolate the potential on capacitor C, from that on capacitor C,. Without the grid the capacitance between the cathode and screen would tend to equalize the potentials of the two electrodes. This amplification can be shown as follows: Initially, capacitor C, is charged to a value Q by piezoelectric action. Therefore, before charging C, and after charging C,, Q, C, V, and Q, 0. After the charge is removed from C, and deposited on C,, then C, V, Q, and C, V, E O and since all or nearly all of the charge is transferred then Q, Q,. The capacitance of C, is selected to be less than the capacitance of C, (C, C,) and since Q, Q, therefore C, V, C, V, and hence V, V thereby resulting in amplification.

Referring to FIGS. 2 and 3, electron gun 51 includes cathode 53 and anode 55 that provides a beam of electrons. The electron beam passes through electric or magnetic deflection device 57 for deflecting the beam for scan and destructive readout. The collector electrode ring 59, or a coarse grid, is provided between mosaic screen 47 and the electron gun. The collector may also be the first stage of a secondary electron multiplier.

During the scan, two occurrences happen simultaneously and these are based on the phenomenon of secondary emission. Secondary emission is the emission of electrons from a surface as a result of that surface being bombarded by a stream of electrons. The secondary electron emission ratio is defined as n,/n, where n, is the number of secondary electrons and n, is the number of primary electrons or the number of electrons in the incident beam. This relationship is further illustrated by the curve shown in FIG. 8 where the ordinate represents the secondary electron emission ratio n,/n, and the abscissa represents the electron energy (shown in units of electron volts e") of an electron impinging upon the surface in question and is a function of the electron velocity V. Point A of this curve represents the critical potential where n,ln, 1. Below this potential n,/n, l and between point A and point B n,/n, 1. This means that electrons can be removed from a surface by bombarding the surface with an electron beam having electrons above the critcal potential, and can add electrons to a surface by bombarding it with an electron beam having electrons below the critical potential. Therefore, if the voltage on the gun anode is greater than the critical voltage, then the number of electrons leaving the bombarded surface will be greater than the number arriving so that the surface will continuously lose electrons and move towards a positive potential and will continue until it reaches approximately the anode potential of the gun.

In accordance with the present invention, the electron beam leaving the gun is above the critical potential of the screen material. Part of this above-criticalpotential beam strikes elements 49 of mosaic screen 47 (forming one plate of C,) and part of the beam passes therethrough and through grid 45, where the beam is decelerated below critical potential, and then strikes elements 43 of photocathode mosaic (forming one plate of C,). Assuming point A of FIG. 7 is the potential of capacitor C after receiving electrons from the photocathode, then capacitor C, will recharge along the dotted line to the original positive potential, due to the net loss of electrons by secondary emission from elements 49 of mosaic screen 47, when bombarded by an electron beam above critical potential. Conversely, since capacitor C, (or elements 43 thereof) had previously lost electrons, it is now necessary to add electrons to bring it back to its original state. This is achieved by the net addition of electrons to elements 43 by virtue of the electron beam being at less than the critical potential. It can therefore be seen that a single beam simultaneously provides the necessary scanning of capacitors C, and C to return them to their original states and also provides readout of capacitors C either by sensing the current flow through resistor R or the current flow collected by the collector electrode 59.

In a typical embodiment of the present invention the distance between photocathode mosaic 41 and grid 45 in about 0.02 inch, between grid 45 and mosaic screen 47 about 0.1 inch, and between mosaic screen 47 and collector electrode 59 about 0.5 inch.

In FIG. 9 is illustrated another embodiment of the present invention. In the FIG. embodiment there is no mosaic screen as illustrated in the FIG. 2 embodiment by reference numeral 47. The FIG. 9 embodiment includes elements 43 mounted in a transducer element and photocathode mosaic similar to that described in relation to the FIG. 2 embodiment. Also included in the FIG. 9 embodiment is screen 45, electron gun cathode 53 and anode 55, collector electrode 59 and a deflection device and light source not shown. The operation of this embodiment is as follows. During that period when the peizoelectric voltage E, is in the negative half cycle, electrons will be emitted from element 43 and collected by grid 45. Capacitor C, will charge as indicated over the time duration of the received echo signal. After the charging of capacitor C, then it is discharged by the electron gun beam by electrons striking it at a potential less than the critical potential. This constitutes the destructive readout wherein the collected charge on capacitor C, is measured by the readout voltage V, across resistor R It is to be understood that there are various alternative techniques that could be made in practicing the present invention by those skilled in the art without departing from the scope of the present invention. For example, an electron multiplier could be used in place of the collector electrode in order to amplify the secondary electron current. Also, the construction of the mosaic screen could be similar to the deposited film type used in image orthicon tubes as long as the electron beam was provided sufficient access through it to discharge the photocathode during the scan mode. Another modification is to replace the piezoelectric face plate with an insulating plate pierced by a mosaic array of individual conductors. The conductor mosaic allows any set of electrical signals to be introduced through the face in a twodimensional array. The unique features of signal gating and storage of the present invention could then be accomplished by coating the inner ends of the conductors with photoemissive material to produce a mosaic of photocathodes as described above. The tube need not be constructed of glass, but could be of metal-ceramic construction for improved mechanical ruggedness. Furthermore, associated electronic circuits such as power supplies and amplifiers could be mounted inside the vacuum envelope to minimize the number of insulated pins penetrating the envelope and/or to confine high voltages to within the vacuum region. Such a construction would be of advantage for underwater or high altitude applications. Moreover, charging currents to C, and C and the secondary current might also be added or multiplied to provide correlation of the video signals as a signal processing technique to further reduce noise. It should also be noted that readout of the FIG. 2 embodiment can be made across a resistor connected in the ground circuit of capacitor C, rather than in the ground circuit of capacitor C What is claimed is:

1. An electron tube device comprising:

a. a transducer element including a plate having first and second sides;

b. said first side of said plate being coated with electrical conducting material;

0. said second side of said plate being coated with a first mosaic of photoemissive material;

d. a light source for illuminating said first mosaic;

e. said first mosaic comprising a plurality of first capacitive elements wherein each of said first capacitive elements is electrically isolated from adjacent capacitive elements;

f. first means for removing electrons from said plurality of capacitive elements of said first mosaic; and

g. second means for storing the number of electrons removed from each of said capacitive elements.

2. The device of claim 1 wherein a. said first and second means and said first mosaic are enclosed in an evacuated envelope.

3. The device of claim 2 wherein:

a. said first means comprises a grid at a positive potential with respect to said first mosaic 4. The device of claim 3 including:

a. said second means comprises a mosaic screen including a plurality of second capacitive elements wherein each of said second capacitive elements is electrically isolated from adjacent elements; and

b. said grid being positioned between said first mosaic and said mosaic screen.

5. The device of claim 4 wherein:

a. an electron gun generating an electron beam for scanning said elements of said first mosaic and for scanning said elements of said mosaic screen.

6. The device of claim 5 wherein:

a. said electron beam being discharged from said gun at a potential above the critical voltage of secondary electron emission from said mosaic screen material;

b. part of the electrons of said beam striking said elements of said mosaic screen thereby providing a secondary emission of electrons from these elements that exceeds the number of electrons cap- 3,742,284 9 10 a. the capacitance of each of said first capacitive eleement to said second capacitive element results in ments is greater than the capacitance of each of a greater voltage on said second capacitive element said second capacitive elements; whereby than the voltage on said first capacitive element. b. the charge transferred from said first capacitive el- 

1. An electron tube device comprising: a. a transducer element including a plate having first and second sides; b. said first side of said plate being coated with electrical conducting material; c. said second side of said plate being coated with a first mosaic of photoemissive material; d. a light source for illuminating said first mosaic; e. said first mosaic comprising a plurality of first capacitive elements wherein each of said first capacitive elements is electrically isolated from adjacent capacitive elements; f. first means for removing electrons from said plurality of capacitive elements of said first mosaic; and g. second means for storing the number of electrons removed from each of said capacitive elements.
 2. The device of claim 1 wherein a. said first and second means and said first mosaic are enclosed in an evacuated envelope.
 3. The device of claim 2 wherein: a. said first means comprises a grid at a positive potential with respect to said first mosaic
 4. The device of claim 3 including: a. said second means comprises a mosaic screen including a plurality of second capacitive elements wherein each of said second capacitive elements is electrically isolated from adjacent elements; and b. said grid being positioned between said first mosaic and said mosaic screen.
 5. The device of claim 4 wherein: a. an electron gun generating an electron beam for scanning said elements of said first mosaic and for scanning said elements of said mosaic screen.
 6. The device of claim 5 wherein: a. said electron beam being discharged from said gun at a potential above the critical voltage of secondary electron emission from said mosaic screen material; b. part of the electrons of said beam striking said elements of said mosaic screen thereby providing a secondary emission of electrons from these elements that exceeds the number of electrons captured by these elements; c. the other part of said beam passing through said mosaic screen and then through said grid and then striking said elements of said first mosaic; d. said grid slowing said other part of said electron beam below the critical voltage; whereby e. secondary emission of electrons from the elements of said first mosaic is less than the number of electrons captured by these elements.
 7. The device of claim 4 wherein: a. the capacitance of each of said first capacitive elements is greater than the capacitance of each of said second capacitive elements; whereby b. the charge transferred from said first capacitive element to said second capacitive element results in a greater voltage on said second capacitive element than the voltage on said first capacitive element. 